I have an honest question. As far as I know, Nichiren Buddhism promises the attainment of Buddhahood in one lifetime. Are there any living Buddhas today who attained Buddhahood through chanting the Lotus Sutra?

In the Lotus Sutra itself, Bodhisattvas, with the exception of the Dragon King's daughter, take many lifetimes honoring many Buddhas before they attain Buddhahood themselves.

If, in the presence of the Buddha any beings such as these hear a single verse or line of the Lotus Sutra, and thereupon have even one thought of rejoicing in it, I will bestow upon them my prediction that they will attain highest, complete enlightenment...

If, after the parinirvāṇa of the Tathāgata, any being hears even a single verse or line of the Lotus Sutra, and thereupon has even one thought of rejoicing in it, I will bestow upon them the prediction that they will attain highest, complete enlightenment.

If there is anyone who preserves, recites, explains, or copies even a single verse of the Lotus Sutra, or who respects this sutra as if it were a buddha, or who reverently offers it various flowers, perfumes, necklaces, fragrant ointments, scented powders, burning incense, canopies, flags, banners, clothing, or music, or who simply honors it with his palms pressed together, know, O Bhaiṣa jyarāja, that this person has already paid homage to tens of myriads of koṭis of buddhas of the past! Such people have completed their great vow in the presence of all the buddhas and yet they have been born as humans out of their compassion for sentient beings.

“O Bhaiṣajyarāja! If anyone should ask you what kind of sentient being will become a buddha in the future, you should inform them that it is those of this kind who will definitely become buddhas in the future. Why is this?

“If there are any sons or daughters of a virtuous family who preserve, recite, explain, and copy even a single line of the Lotus Sutra, or who pay homage to this sutra with various offerings of flowers, perfumes, necklaces, scented powders and ointments, burning incense, canopies, flags, banners, clothing, or music, or who honor it with their palms pressed together, such people should be respected by the entire world. They should be revered in the same way as the Tathāgata is revered. Know that these people are great bodhisattvas who are to attain highest, complete enlightenment. Out of their compassion for sentient beings they wish to be born among them in order to expound and explain the Lotus Sutra far and wide. How much more to be honored are those who completely preserve the entire sutra and pay homage to it with various offerings!

“O Bhaiṣajyarāja! You should know that after my parinirvāṇa these people will set aside the rewards of their pure karma and be born in the troubled world out of their compassion for sentient beings, and in order to expound this sutra far and wide. After my parinirvāṇa, if there are any sons and daughters of a virtuous family who expound even a single line of the Lotus Sutra in private to even a single person, they should be acknowledged as the ambassadors of the Tathāgata. They have been dispatched by the Tathāgata and carry out the Tathāgata’s work. As for those who extensively teach among the common people, know that they are yet greater ambassadors.

“O Bhaiṣajyarāja, if there are erring people during a kalpa, those with troubled thoughts who always disparage the Buddha in his presence, their fault is nonetheless slight. Yet if there are those who disparage the laity or ordained people who recite the Lotus Sutra even with a single evil word, their fault is extremely grave.

“O Bhaiṣajyarāja! Know that those who recite this Lotus Sutra adorn themselves with the adornments of a buddha. This means that they will carry the Tathāgata on their shoulders and pay him homage wherever they go. They should be respected, revered, honored, and praised wholeheartedly with palms pressed together, through offerings of flowers, perfumes, necklaces, scented powders, ointments, burning incense, canopies, flags, banners, clothing, delicious food, music, and the best offerings that people can make. They should have heavenly jewels scattered upon them and offered to them. Why is this? Because these people joyfully expound the Dharma and those who hear it even for an instant will fully attain highest, complete enlightenment.”

“Once you have given up the ghost, everything follows with dead certainty, even in the midst of chaos.”
-Henry Miller

"Language is the liquid that we're all dissolved in.
Great for solving problems, after it creates the problems."
-Modest Mouse

"Wake up to find out that you are the eyes of the world!"
-The Grateful Dead

The assurances and predictions in the Lotus Sutra are not that one will instantly attain Buddhahood, nor will even do so in one lifetime. If you look at the passages in context, they are assuring that all those who set out on the Bodhisattva path, practicing the Six Paramitas, will attain Buddhahood no matter how many lifetimes (possibly eons) it takes.

The Six Paramitas are
1. OFFERING (your offering to the Buddha, Dharma, Sangha, and your ancestors),
2. PRECEPT (to practice rules as Buddhists, in Nichiren Buddhism it is to chant “Namu
Myo-ho Ren-ge Kyo”),
3. PATIENCE (not to get upset, try to control your lust, to bear hardship),
4. DEVOTION (your daily practice in Buddhism),
5. MEDITATION (to maintain calmness or quiet state of your mind) and
6. BUDDHA WISDOM (attaining the reality of all things).
Nichiren Shonin gave us a remedy, “NAMU MYO-HO REN-GE KYO.”

You don’t need to make effort to practice the above mentioned Six Paramitas. Why is that? It
is because when you chant it, you will act the Six Paramitas without knowing. You will
become patient, be able to overcome hardship, get some sort of wisdom because your
mind is calm.http://www.nichiren-shu.org/LasVegas2/P ... NCE_14.pdf

You seem to assume there is something to attain. There is nothing to attain.

O Śāriputra! You should now know
That originally I made a vow
To make all sentient beings my equal
Without any difference.
Now I have already fulfilled this vow
That I made in the past...
Even though I taught nirvana,
It is not the true extinction.
Every existing thing from the very beginning
Has always had the mark of quiescence.

The problem that some bodhisattvas have is that they are attached to the six paramitas. They think that they have to toil away for eons to earn Buddhahood. This is an upaya for those who can't believe they already are Buddha and can instantaneously manifest Buddhahood like the dragon girl. Just like the poor son who thinks he deserves nothing better than to scrub toilets and earn minimum wage. The father can't convince the son otherwise, so the father indulges him by giving him the job he thinks he deserves.

The Bodhisattvas who understand the ekayana, still practice the paramitas because they are playing their part in the upaya. Sometimes, they appear as Samyaksambuddhas and turn the wheel anew - they make a show of being born, leaving home, sitting under a tree and attaining annuttarasamyaksambodhi, turning the wheel and entering parinirvana. Its all very much like a passion play. Its a show for beings who don't get it yet.

“Once you have given up the ghost, everything follows with dead certainty, even in the midst of chaos.”
-Henry Miller

"Language is the liquid that we're all dissolved in.
Great for solving problems, after it creates the problems."
-Modest Mouse

"Wake up to find out that you are the eyes of the world!"
-The Grateful Dead

Dharma Flower wrote:This Nichiren Buddhist article is in favor of the Six Paramitas:

The Six Paramitas are
1. OFFERING (your offering to the Buddha, Dharma, Sangha, and your ancestors),
2. PRECEPT (to practice rules as Buddhists, in Nichiren Buddhism it is to chant “Namu
Myo-ho Ren-ge Kyo”),
3. PATIENCE (not to get upset, try to control your lust, to bear hardship),
4. DEVOTION (your daily practice in Buddhism),
5. MEDITATION (to maintain calmness or quiet state of your mind) and
6. BUDDHA WISDOM (attaining the reality of all things).
Nichiren Shonin gave us a remedy, “NAMU MYO-HO REN-GE KYO.”

You don’t need to make effort to practice the above mentioned Six Paramitas. Why is that? It
is because when you chant it, you will act the Six Paramitas without knowing. You will
become patient, be able to overcome hardship, get some sort of wisdom because your
mind is calm.http://www.nichiren-shu.org/LasVegas2/P ... NCE_14.pdf

Sure.

Nichiren Shu's doctrines are not universally accepted. Views expressed by Nichiren Shu temples are not universal even among all who are nominally part of Nichiren Shu. Nichiren Shu is an umbrella organization that more or less exists because at one point it was required under Japanese law. Its substantively different than other Nichiren sects which are organized around more specific sets of doctrines and rituals. Sort of complicated story.

“Once you have given up the ghost, everything follows with dead certainty, even in the midst of chaos.”
-Henry Miller

"Language is the liquid that we're all dissolved in.
Great for solving problems, after it creates the problems."
-Modest Mouse

"Wake up to find out that you are the eyes of the world!"
-The Grateful Dead

Queequeg wrote:
The problem that some bodhisattvas have is that they are attached to the six paramitas. They think that they have to toil away for eons to earn Buddhahood. This is an upaya for those who can't believe they already are Buddha and can instantaneously manifest Buddhahood like the dragon girl.

Not everyone sees practicing the Six Paramitas as "earning" Buddhahood:

The Six Perfections describe the true nature of an enlightened being, which, in Mahayana practice, is to say they are our own true buddha-nature. If they don't seem to be our true nature, it is because the perfections are obscured by our delusion, anger, greed, and fear.

Are there any commentaries outside of Nichiren Buddhism, including Tendai, which make this claim?

You could look at the Sutra itself, at the context of the quote you lifted. Maitreya asks Manjusri about the significance of the miraculous light emitted from the Buddha's urna (tuft of hair between the eyes). Manjusri then recounts the last time he saw it. He describes the teaching of the the Buddha Candrasuryadipa:

For the sake of those seeking to become voice-hearers he responded by expounding the Law of the four noble truths, so that they could transcend birth, old age, sickness and death and attain nirvana. For the sake of those seeking to become pratyekabuddhas he responded by expounding the Law of the twelve-linked chain of causation. For the sake of the bodhisattvas he responded by expounding the six paramitas, causing them to gain anuttara-samyak-sambodhi and to acquire the wisdom that embraces all species."

Eventually, this Buddha shoots light out of his urna then teaches the Lotus Sutra.

The Buddha goes on to explain in the Lotus Sutra that the Lotus Sutra is the Buddha's highest teaching and that everything else preceding the teaching of the Lotus Sutra is upaya, preparing for the revelation of the Lotus Sutra.

Is there anywhere in the sutra itself which endorses chanting its title as the true cause of Buddhahood?

Throughout the Sutra, the assembly is instructed to recite the Sutra. The Lotus Sutra is the universal gate of Buddhahood - the ekayana.

Reciting the title is reciting the Sutra.

“Once you have given up the ghost, everything follows with dead certainty, even in the midst of chaos.”
-Henry Miller

"Language is the liquid that we're all dissolved in.
Great for solving problems, after it creates the problems."
-Modest Mouse

"Wake up to find out that you are the eyes of the world!"
-The Grateful Dead

Queequeg wrote:
The Buddha goes on to explain in the Lotus Sutra that the Lotus Sutra is the Buddha's highest teaching and that everything else preceding the teaching of the Lotus Sutra is upaya, preparing for the revelation of the Lotus Sutra.

Please forgive me if I'm wrong. Does Nichiren Buddhism teach that everything in the Lotus Sutra other than the 15th through 28th chapters are provisional?

Queequeg wrote:
The problem that some bodhisattvas have is that they are attached to the six paramitas. They think that they have to toil away for eons to earn Buddhahood. This is an upaya for those who can't believe they already are Buddha and can instantaneously manifest Buddhahood like the dragon girl.

Not everyone sees practicing the Six Paramitas as "earning" Buddhahood:

Yep. That's what I wrote. You're trying to make controversy where there is none.

While everyone has Buddha-nature, it has to be realized in order to attain Buddhahood.

Nothing to be realized. Buddha-nature is irrepressible.

“Once you have given up the ghost, everything follows with dead certainty, even in the midst of chaos.”
-Henry Miller

"Language is the liquid that we're all dissolved in.
Great for solving problems, after it creates the problems."
-Modest Mouse

"Wake up to find out that you are the eyes of the world!"
-The Grateful Dead